
























COPYRIGHT DEPOSIT. 









I • 


VALVE GEARS 


INSTRUCTION PAPER 


PREPARED BY 

Walter S. Leland, S.B. 

A 

Society Naval Architects and Marine Engineers. 
Assistant Professor of Naval Architecture 
Massachusetts Institute of Technology. 


AMERICAN SCHOOL OF CORRESPONDENCE 


CHICAGO 


U. S. A. 


ILLINOIS 















LIBRARY of CONGRESS 
Two Copies Received 

NOV 2 1906 



Copyright 1906 by 

American Schooe of Correspondence 


Entered at Stationers’ Hall. London 
All Rights Reserved 







VALVE GEARS, 


Steam enters the cylinder of the engine through ports which 
must, in some manner, be opened and closed alternately, in order 
to admit and exhaust the steam at the proper time. To accom¬ 
plish this purpose a valve is moved back and forth across the port 
openings. A complete understanding of the valve and valve gear 
is essential to the engineer as well as to the designer, for even 
though a valve be properly designed, its economy may be seri¬ 
ously impaired by improper setting. The design and adjust¬ 
ment of these valves plays a very important part in the efficient 
action of the steam engine. 

The term “valve gear” includes the valve or valves that 
admit steam to and exhaust it from the cylinder of the engine, 
together with the mechanism from which the valves derive motion. 
There may be a single valve to regulate admission and exhaust, or 
there may be a double set of valves; one set to admit the steam at 
each end and another to release it. The valve may have a plain 
reciprocating motion, moved 
by a rod, or it may be opened 
by some device that lets go at 
the proper time, allowing the 
valve to drop shut under the 
influence of counter weights, 
springs or vacuum dashpots. 

To the first class belong the Fig, 1. 

plain slide valve and its modi¬ 
fication of piston valve, gridiron valve, etc.; to the second belong 
such valves as the Corliss, Brown, and others. 

The simplest type of valve is the plain slide or D valve as 
shown in Fig. 1. 

In this figure Y is the valve, R the valve rod, K the exhaust 
cavity, P and P' the steam porta, E the exhaust port, AB the valve 
seat, and DM the bridges of the valve seat. The valve seat must 
be planed perfectly smooth, so that pressure on the valve will 
















4 


VALVE GEARS 


make a steam tight fit, and cause as little friction as possible when 
the valve slides. Furthermore, the length of the seat AB must be 
a little less than the distance from the extreme right-hand posi¬ 
tion of the right-hand edge of the valve to the extreme left-hand 
position of the left-hand edge of the valve. This allows the valve at 
each stroke slightly to over travel the seat, thus keeping it always 
worn perfectly flat and smooth. If the valve seat were not raised 
slightly above the rest of the casting, or if it were too short, the 
constant motion of the valve would soon wear a hollow path in the 
valve seat, and it would cease to be steam tight. 

Eccentric. The valve usually receives its motion from an 
eccentric which is simply a disc, keyed to the shaft in such a 



manner that the center of the disc and the center of the shaft do 
not coincide. Ij is evident that as the shaft revolves, the center 
of this eccentric disc moves in a circle about the shaft as a center, 
just as if it were at the end of a crank. The action of the eccentric 
is equivalent to the action of a crank the length of which is equal 
to the eccentricity of the eccentric (the distance between the center 
of the eccentric and that of the shaft). 

Fig. 2 represents the essentials of an ordinary eccentric. O 
is the center of the shaft, O the center of the eccentric disc E, and 
S is a collar encircling the eccentric and attached to the valve 
rod R. 

































VALVE GEARS 


5 


As the eccentric turns in the strap, the point O moves in the 
uotted circle around O', and the point A also moves in a circle. 
When half a revolution is accomplished the point O will be at 
O", the point A will be at A", and the eccentric strap and valve 
rod will be in the position indicated by the dotted lines. 

Since the revolving shaft transmits motion to the valve 
through the eccentric, it will be necessary to study the relative 
motions of the crank and eccentric in order to get a clear idea of 
the steam distribution. 

The distance of the center of the eccentric from the center 
of the shaft (00' in Fig. 2) is known as the eccentricity, or throw, 
of the eccentric. The travel of the valve is twice the eccentricity. 

Valve without Lap. Fig. 3 shows a section through the 
steam and exhaust ports of an engine, together with a plain slide 



valve placed in mid-position, and so constructed that in this posi¬ 
tion it just covers the steam ports and no more. A valve is in 
mid-position when the center line of the valve coincides with the 
center line of the exhaust port. 

Fig. 1 shows the same valve drawn to a larger scale. 

Suppose the valve is moved a slight distance to the right; 
the port P (see Fig. 1) is then uncovered and opened to the live 
steam which enters the cylinder and causes the piston to move. 
Since the two faces of the valve are just sufficient to cover the 
steam ports, it is evident that as the port P opens to live steam, 
the port P' opens to the exhaust. The ports are closed only when 
the valve is in mid-position. This allows admission and exhaust 
























6 


VALVE GEARS 


to continue during the whole stroke. With such a valve there is 
no expansion or compression; the indicator card would be a rec¬ 
tangle, and the M. E. P. would be equal to the initial steam pressure, 
assuming no frictional losses in the steam pipe or condensation in 
the cylinder. 

For a theoretical discussion of valve motion, it is assumed that 
the eccentric rod moves back and forth in a line parallel to the 
center line of the engine. This is not the case in practice, for the 
eccentric rod always makes a small angle with the center line, just 
as the connecting rod does, but the eccentricity is so small in com¬ 
parison with the length of the eccentric rod that the angularity of 
the eccentric rod is very much smaller than the angularity of the 
connecting rod, and its influence may be neglected without appre¬ 
ciable error. 




When the valve shown in Fig. 3 is in mid-position, the crank 
is on dead center, the eccentric is set at right angles to it, and the 
piston is just ready to begin the stroke. 

Fig. 4 shows the relative positions of crank, piston, eccentric 
and valve when the crank has made a quarter turn or the piston 
has moved to half stroke. The eccentric is now in its extreme 
position to the right, the valve has its maximum displacement and 
both the steam and exhaust ports are wide open. The valve will 
not close again until the piston has reached the end of its stroke. 

This type of valve is used only on small and unimportant 
engines, and since it allows no expansion of the steam, is very 
uneconomical. Furthermore, it will be seen that this valve opens 
just after the stroke begins, which is impractical, for it means that 
the piston has begun its ^ stroke before the full steam pressure 

































VALVE GEARS 


7 


reaches it, which will cause an inclined admission line on the indi¬ 
cator diagram. 

Valve with Lap. If the face of the valve is made longer 
than shown in Fig. 1, so that in mid-position it overlaps the steam 
ports, we shall have a valve such as shown in Fig. 5. The 
amount that the valve overlaps the steam ports is called the lap of 
the valve. In Fig. 5, DI is the inside lap, and OC the outside lap. 
It will at once be seen that both the admission and exhaust ports may 
remain closed during a part of the stroke, thus making expansion 



and compression possible. It is also evident that steam cannot 
be admitted until the valve uncovers the port by moving a dis¬ 
tance from mid-position equal to OC. Admission continues until 
the valve returns to such a position that the outer edge of the 
valve again closes the port. Release will begin when the inner 
edge of the inside lap begins to uncover the port. 

Fig. 6 represents a valve with lap, at the point of admission. 
Since the valve must move a distance equal to the outside lap 
before admission can take place, it is evident that the eccentric can 
no longer be at right angles to the crank at the beginning of the 
stroke, but must be ahead of the right-angle point by an amount 
equal to AOC. The angle AOC is known as the angular advance. 

The maximum displacement of the valve is attained when 
the eccentric is horizontal as shown in Fig. 7. In this position 


























































8 


VALVE GEARS 


both the steam and exhaust ports are wide open, and any further 
motion of the piston will cause the valve to move toward its mid¬ 
position. 

Admission continues until the valve returns to the position 



shown in Fig. 8. Here the outside lap just closes the left-hand 
ste^m port, cut-off takes place, and the steam already in the cylin¬ 
der begins to expand. As the valve continues to move toward the 
left, the left-hand inside lap begins to uncover the left-hand port 
and releases the steam at the position shown in Fig, 10, 

The dotted lines of Fig. 7 show the valve in its extreme 



position to the left. Any further movement of the piston will 
cause it to return toward mid-position. 

The dotted position of crank and eccentric in Fig. 10 shows 
the valve returned to the point of compression, which continues 
until the conditions of Fig. 6 are again reached and the opening 
valve allows steam again to enter the cylinder. 

This process has been traced step by step for one end only; 
let us now consider what is happening at the other end. 



















































VALVE GEARS 


9 


Admission is the point at which the valve opens to admit 
steam to the cylinder. Cut=off is the point at which the valve 
closes to cut off the admission of steam. Release is the point at 
which the exhaust is opened ; and Compression is the point at 
which the exhaust is closed. 



While the crank is moving from the position shown in Fig. 
0 to that of Fig. 8, steam is being admitted to the head end and 
being exhausted from the crank end. The inside lap being less 
than the outside lap, causes the exhaust to continue longer than 
the admission. 

Fig. 9 shows the relative positions of crank, eccentric and 
valve when the exhaust closes on the crank end and compression 



begins. Between these two positions the steam is expanding in 
the head end and exhausting from the crank end. 

Between the positions of Fig. 9 and Fig. 10 both ports are 
entirely closed, expansion is taking place in the head end and com¬ 
pression in the crank end. Fig. 10 is head-end release. Fig. 11 
shows admission at crank end of cylinder and marks the end of 
crank-end compression. 































































10 


VALVE GEARS 


By referring to Figs. 6-11, the effect of any change of lap 
may at once be observed. If the outside lap is increased , the 
valve must move farther from mid-position before admission will 
occur, and on the return, after the maximum displacement is 
reached, the outside lap, being wider, will close the port sooner, 
and the cut-off shown in Fig. 8 will take place before the crank 



reaches the angle there shown. A decrease of outside lap will 
make cut-off later and admission earlier. 

If the inside lap is increased, the valve must move farther be¬ 
fore release occurs and the crank angle would be greater than shown 
in Fig. 10. On the return of the valve to the dotted position shown 
in Fig. 10, the port will close earlier and make an earlier compres-* 
sion; the crank angle will be less than is there shown. Decreasing 
inside lap will cause earlier release and later compression. 




Thus we see that it is the outside lap that influences admis¬ 
sion and cut-off, and the inside lap that controls release and 
compression. For this reason the outside lap is often called the 
steam laj), and the inside lap the exhaust lap. 































































VALVE GEARS 


11 


Leadc If a valve having lap is in mid-position, the port is 
closed and the engine cannot start because no steam can enter the 
cylinder. That the steam may be ready to enter the cylinder at 
the beginning of the stroke it is necessary that the eccentric be 
set more than 90° ahead of the crank and the eccentric radius will 
take an angle as shown in Fig. 6, called the angular advance. In 
order that the ports and 
clearance may be prop¬ 
erly tilled with steam 
at the beginning of the 
stroke, it is necessary 
that the valve be dis-. 
placed from its mid- 
position an amount 
slightly greater than 
the outside lap. With 
the piston at the end of the stroke the valve will have a position as 
shown in Fig. 12. The port will be open the distance AB. This 
causes the eccentric to be moved forward a slight amount in excess 
of the angular advance. This excess is called the angle of lead. 

In Fig. 13, O'R' represents 
the crank at the beginning of the 
stroke, LOA the angular ad¬ 
vance, and AOA' the angle of 
lead. The eccentric, to give 
lead, must be set at the angle 
110A' ahead of the crank or 90° 
plus angular advance plus angle 
of lead. In large, quick-run¬ 
ning engines, a liberal lead is 
essential, so that the ports and 
clearance may be well filled 
with steam before the stroke 
begms. If there is no lead, a 
portion of the steam will be used 
in filling these places and full 
pressure steam will not reach the piston until it is well advanced 
on the stroke. This will give a sloping admission line as <diQwu 



Fig. 13. 






















12 


VALVE GEARS 


in Fig, 14. Too much lead, on the other hand, will cause too 
early an admission as shown in Fig 15. 

If the angular advance is increased, the eccentric will be 
moved farther ahead of the crank, and consequently will begin its 
motion sooner. It will necessarily arrive at each of the events 


Fig 14. Fig. 15. 

sooner than before. If then, the angular advance is increased, all 
'vf the events of the stroke will occur earlier. 

Inequality of Steam Distribution. In the valve diagrams 
thus far considered, the events of the stroke have been discussed 
for each end separately, without reference to the relation of sim¬ 
ilar events on the other side of the 
piston. If the connecting rod 
were of infinite length, so that it 
would always remain parallel to 
the center line of the engine, the 
distribution would be the same 
for both ends of the cylinder. In 
practice, the connecting rod is 
from 4 to 8 times the length of 
the crank, which causes the con¬ 
necting rod always to be at an 
angle to the center line of the 
engine, and for a given crank 
angle makes the piston displace¬ 
ment greater at the head end than at the crank end. 

To find the displacement of the valve , let us consider Fig. 16. 
The circle represents the path of the eccentric center during a 
complete revolution of the engine. OC represents the crank, and 
OR the corresponding position of the eccentric. The diameter 
XY represents the extent of the valve travel. Since the eccentric 
rod is so long in comparison to the eccentricity, we make no 
appreciable error by assuming it always to be parallel to the center 



















VALVE GEARS 


13 


line of the engine. When the eccentric is at OL, the valve is in mid- 
position. At OR the valve has moved from mid-position an amount 
ON, found by dropping a perpendicular from R to the center line 
. If the angularity of the connecting rod could be neglected, 
the piston displacement could be found in the same manner. 

To find the displacement of the piston, a diagram as shown 
in Fig. 17 must be drawn. In this figure AB represents the 
cylinder, P the piston, H the crosshead, HR the connecting rod, 
and OR the crank. Suppose now the engine should stop in this 
position and then be clamped. The piston displacement would 
be represented by AP. If the crank pin at R should now be 
loosened so as to allow the connecting rod to fall to a horizontal posi¬ 
tion, the point R would describe the arc of a circle RH, and XN 
would represent the piston displacement and would be equal to AP 



Suppose now that in this disconnected way the piston, crosshead 
and connecting rod were moved forward until the end of the rod 
came to O. P would then be at P' and the piston would be in the 
middle of its stroke. Now swing the end of the rod up to its 
proper position on the crank-pin circle, the piston remaining sta¬ 
tionary. It would describe an arc OZ. The crank pin would be 
at Z, less than a quarter revolution from X, while the piston would 
be in the middle of its stroke. 

Suppose this engine were running with cut-off at half stroke 
on the head end and that XOZ represented the corresponding crank 
angle. On the return stroke the valve would cut off at the same 
crank angle YOT = XOZ, and OT would represent the crank cut¬ 
off on the return or crank-end stroke. The piston, as we have just 
seen, will not be at half stroke except when the crank is at OZ or OS. 
Consequently OT is less than half stroke and cut-off takes place 
earlier at the crank end than at the head end. When the crank is at 
OZ the eccentric will be at OA (Fig 11a), and the valve displace- 
















14 


VALVE GEARS 


ment will be OB. When the crank is at OT the eccentric will be at 
OA','and the valve displacement will be OB', which is equal to OB, 
the displacement of the valve at cut-off in the head end. The pis¬ 
ton displacement will be OX in the head end and WY in the crank 
end when cut-off occurs. If the connecting rod always remained 
parallel to the center line, the cut-off would be the same at both ends. 

Compensation of Cut=off. It has already been pointed out 
that lengthening the outside lap makes the cut-off earlier, and short¬ 
ening the lap makes it later. The cut-off in the case just cited may 
then be equalized by altering the outside laps. If we increase the 
outside lap on the head end, or decrease the crank-end lap, the 
inequality will be less. By changing either or both of the laps the 
proper amount, the cut-off may be exactly equalized. 



But altering the outside lap changes the lead as has already 
been explained. If the lap is increased on the head end, the lead 
will be less than on the crank end. If the lead becomes too small 
on the head end, the angular advance may be increased but the 
inequality of lead will still remain, for this increase of angular 
advance will increase the lead at the crank end as well as at the 
head end, and by hastening all the events of the stroke may give a 
bad steam distribution if care is not taken. 

Unequal lead is of less consequence on a low-speed than on a 
high-speed engine. On low-speed engines the cut-off may be 
equalized at the expense of lead with beneficial results, but on high¬ 
speed engines it will not do to give too little lead at one end. A 
high-speed engine requires more lead than a low-speed, for there is 
relatively less time in each stroke for the clearance to fill with steam. 






VALVE GEARS 


15 


If both inside laps are equal, compression will not occur 
equally at both ends. To equalize it, the inside laps may be 
changed in the same.manner as the outside laps are changed to 
equalize the cut-off. By altering these inside laps to equalize 
compression, it may happen that the lap is reduced enough to 
leave the exhaust port open when the valve is in mid-position. 



This opening of the valve is called an inside clearance, or negative 
lap. In Fig. 4-8, A is the inside clearance. 

Rocker. Sometimes it happens that the valve stem and 
eccentric rod cannot be so placed that they will be in the same 
straight line; or it may be that the travel of the valve must be so 
great as to require an excessively large eccentric. In such cases 
a rocker may be used. 

Fig. 19 shows a valve that is not in line with the eccentric. 
This occurs in horizontal engines when the valve is set on top ot 



the cylinder instead of on one side. By means of the rocker AG 
the valve may receive its proper motion. 

In case it is more convenient to place the pivot of the rocker 
arm between the connections to the valve stem and those of the 
eccentric rod, such an arrangement as shown in Fig. 20 may be 
used. Here it will be noticed that the valve stem and eccentric 
rod are moving in opposite directions, and to give the valve the 









































VALVE GEARS 


J6 


same motion as in Fig. 19, the eccentric must be moved 180° ahead 
of the position there shown. 

If AB is less than AG, the valve travel will be greater than 
twice the eccentricity, in proportion as AG is greater than AB. 
In all cases the valve travel is to twice the eccentricity as AG is to 
AB. Thus, if the valve travel is 4A inches, AB, 15 inches, and 
15 

AG, 18 inches, then — X inches, will equal twice the 


eccentrici ty. 



Fig. 20. 

A valve gear may be so laid out as to make both the cut-off 
and the lead equal for both ends of the cylinder. This may be 
done by a proper ,porportion between the rocker arms, and a careful 
location of the pivot of the rocker. The eccentric must then be 
set accordingly. In this manner the Straight Line engine equal¬ 
izes the cut-off and lead. A discussion of this method will be 
considered later. 

VALVE DIAGRAMS. 

Zeuner’s- Diagram. In order to study the movements of 
valves, the effect of lap, lead, eccentricity, etc., diagrams of various 
sorts have been devised. By the use of diagrams we may acquire 
a knowledge of valve motion without the complex mathematical 
expressions that such a discussion would entail. The most useful 
of these various diagrams is that devised by Zeuner, and to avoid 
complexity we shall confine ourselves to a discussion of this dia¬ 
gram alone. The eccentric rod is assumed to be of infinite 
length, and the positions of the crank are shown on the diagrams. 
The displacement of the piston can easily be found if the ratio of 
crank to connecting rod is known. 

In Fig. 21 let OY be the eccentricity, then XOY will rep¬ 
resent the valve travel, and the center of the eccentric will move 

















VALVE GEARS 


17 



in the circle XWY. Let OR represent thg position of the 
crank and Or the corresponding position of the eccentric, which 
is 90° + angle of advance 6 ahead of the crank. Draw OW 
perpendicular to XY and lay off from it the angle WOM — 
angle of advance 6 towards the crank. With OM as a diameter, 
construct a circle. OM is equal to the eccentricity, and the^cfrcle 
MPO is known as the valve circle. If OR, the center line of 

w -~ M 

£ O W -s. 7 -° W \ 

4^.0^ t cfo 0 . (<* + $) 

4 XOV* s x 6 R - (fl+4) 

4X0 W r Cj 0 ' 

the crank, cuts this valve circle at P, then OP is equal to the dis- oC- 3 
placement of the valve from mid-position. 

To prove this, draw rS perpendicular to XY. Since Or 
is the position of the eccentric, OS will represent the valve dis¬ 
placement from mid-position. Draw MP. Then by geometry 
OPM is a right angle because it is inscribed in a semicircle. 

OSr is also a right angle ; the two right-angled triangles OSr 
and OMP are equal because they are similar^and have two cor¬ 
responding sides equal. Or = OM, being radii of the same 
circle. But we have seen that OS is equal to the valve displace¬ 
ment, therefore OP is also equal to the valve displacement, for it 
is equal to OS. 

Xow that the truth of our proposition has been proved, let 
us see how we may study the valve motion from such a diagram. 

See Fig. 22. As before, let XY represent the valve travel, then 
the circle XEYF will represent the path of the center of the 
eccentric. Let 6 be the angular advance and lay off EO toward the 
crank, making an angle 0 with the vertical. Produce EO to F, 
and on OE and OF as diameters draw the valve circles as shown. 

Let the outside lap be an amount equal to OV, then with O as a 
center and OY as a radius draw an arc intersecting the upper 
valve circle at Y and K. Lay off OP equal to the inside lap and 









18 


VALVE GEARS 


with O as center and OP as a radius draw an arc intersecting the 
valve circle at P and Q. Draw the crank line AO passing through 
Y. Then, when the crank is in this position, the displacement of 
the valve is equal to OY (the outside lap) and the steam is ready 
to enter the cylinder. This is the position of the crank at admis¬ 
sion, and the crank angle XOA is called the lead angle. The 
valve has lead, therefore the admission takes place before the end 
of the stroke. When th,e crank reaches the position OE, the 
displacement of the valve is equal to OE, the eccentricity, and is 





the maximum displacement. Further motion of the piston causes 
the valve to move toward mid-position until, at the crank position 
OC, the displacement OK is again equal to the outside lap and 
the valve has reached the point of cut-off. When the position OH 
is reached, the crank line is tangent to both valve circles and there 
is no displacement of the valve. At this point the valve is in 
mid-position. 

Further crank movement draws the inside lap toward the 
edge of the exhaust port until, at the crank position OB, the dis¬ 
placement is equal to OP (the inside lap) and release begins. At 
OF the maximum valve displacement is again reached and the 
valve moves in the opposite direction until at OD its displacement 








VALVE GEARS 


19 


from mid-position, is again equal to OQ = OP = tlie inside lap, 
and compression takes place. At OH' the valve is again in mid¬ 
position. At OX the displacement of the valve is 01, but since 
the valve has to move the distance OJ before the port begins to 
open IJ must represent the port opening when the crank is on 
dead center and by definition we know that lead is the amount of 
port opening at this position. Therefore IJ represents the lead. 

At the position R, the port is open an amount equal to TG, 
at E the opening is a maximum equal to EX. At C the opening 
is nothing. If LW represents the total width of the steam port, 
the exhaust port will be open wide when the displacement of the 
valve is equal to OW and it will remain wide open while the crank 
swings from 0)V to OK. 



If the width of steam port in addition to the outside lap were 
laid off on the other valve circle it would fall at E'. For the ad¬ 
mission port to be wide open, the displacement of the valve w T ould 
have to be equal to OE', which is more than the maximum dis¬ 
placement. This shows that in this case the steam port is jiever 
fully open and that the left-hand edge of the valve overlaps the 
right-hand edge of the port by an amount equal to EE' when the 
valve has reached its maximum displacement 






20 


VALVE GEARS 


Fig. 22, with its two valve circles, shows the diagram for the 
head end of the cylinder only. The crank-end diagram would be 
similar except that the laps might not be equal to those of the 
head end. 

We are now in a position to consider more in detail the effect 
of changing in any way either the valve or the setting. Let us con¬ 
sider Fig. 23, which is in every way like Fig. 22 except that all 
unnecessary letters and lines are omitted to avoid confusion. If 
the outside lap is increased an amount equal to !NM, the ad¬ 
mission will take place later, at crank position OA'; the lead will 



be reduced to IG and cut-off will take place earlier at OG'. If 
the outside lap is reduced a like amount the contrary effects will 
be observed. If the inside lap is increased an amount equal to LS, 
the release will take place later at the crank position OB' and com¬ 
pression will take place earlier at OD'. The contrary effect will 
be observed by decreasing the inside lap. 

If the angular advance is increased (see Fig. 24) all the events 
will occur earlier. This is evident from the figure; the crank 
revolves in the direction indicated by the arrow and O A' (new posi¬ 
tion of admission) is ahead of OA, the old position. 









VALVE GEARS 


21 


If the eccentricity is increased, Fig. 25, the valve travel will 
increase and admission will take place earlier at OA'; the lead 
will be increased an amount equal to II', and cut-off will take 
place later at OC'. Release will he earlier at OB' and com¬ 



pression will be later at OD'. The upper valve circle will now 
cut the arc drawn from O as a center, with a radius equal to the 
outside lap plus the width of steam port, in the points AV' and H', 
and the admission port will be open wide while the crank is mov¬ 
ing from OW' to OH'. Similarly, the lower valve circle cuts the 
arc drawn from O as a center, with a radius equal to the inside 
lap plus the width of steam port, in the points AV and H. The 
steam port is then wide open to exhaust while the crank is moving 
from AV to H. From the above it will be seen that the periods 
are all changed by changing the travel; thus, admission and ex¬ 
haust begin sooner and last longer, while expansion and com¬ 
pression begin later and cease sooner. AVith change in the angular 
advance, however (see Fig. 24), the periods are neither increased 
nor decreased. 










22 


VALVE GEARS 


For convenience, these results are collected in the following 
table which shows the effect of changing the laps, travel, and 
angular advance: 



Increasing 
Outside Lap. 

Increasing 
Inside Lap. 

Increasing 
. Travel. 

Increasing 
Angular Advance. 

Admission. 

Is later. 

Ceases sooner. 

Not changed. 

Begins earlier. 
Continues 

longer. 

Begins earlier. 
Same period. 

Expansion. 

Is earlier. 
Continues 

longer. 

Beginning 

unchanged. 

Continues 

longer. 

Begins later. 
Ceases sooner. 

Begins earlier. 
Same period. 

Exhaust. 

Unchanged. 

Occurs later. 
Ceases sooner. 

Begins earlier. 
Ceases later. 

Begins earlier. 
Same period. 

Compression 

Begins at same 
point. 

Begins sooner. 
Continues 

longer. 

Begins later. 
Ceases sooner. 

Begins earlier. 
Same period. 


PROBLEMS. 

All the problems on valve gears involve the relations between 
certain variables which are : 


The valve travel. 

Angle of lead. 

Outside lap. 

Inside lap. 

Points of stroke at which admission cut-off, release and compression 
take place. 

In designing a Slide Valve, a few of these variables depend 
upon the conditions under which the engine is to run. For instance, 
the valve travel is limited, cut-off must be at a certain point and 
the engine must have a certain lead. Then, with the aid of a Zeun- 
er’s diagram, the remaining proportions of the valve may be deter¬ 
mined. 

Let us consider a few examples: 

Given the valve travel = 3 inches. 

Inside lap = % inch. 

Angular advance = 35° 

Angle at eut-ofl — 115 u 
















VALVE GEARS 


23 


To determine the laps, the lead and the crank angles at 
admission, compression and release. 

In lig. 26, let XY represent the valve travel = 3 inches. 
Draw OM perpendicular to XY, and on XY as a diameter draw 
the circle XMA F representing the path of the center of the eccen¬ 
tric as it revolves about the shaft. Lay off the angle MOE = the 
angular advance = 35° so that the angle XOE is equal to 90° 





Fig. 26. 


minus the angular advance* Produce EO to F. Then on OE 
and OF as diameters draw the valve circles. The eccentricity 
OE or OF, if no rocker is used, will be half the valve travel. Lay 
off the crank angle XOC = angle of crank at cut-off = 115°, and- 
OK will then represent the distance of the valve from mid-posi¬ 
tion when cut-off takes place. This distance we know is the out¬ 
side lap. Draw the arc KI, known as the lap circle, and it will 
cut the valve circle again at Y. When the valve is again the dis¬ 
tance OY = the outside lap from mid-position, admission will take 
place. Draw the line OYA and this will represent the position 
of the crank at admission. 













24 


VALVE GEARS 


When the crank is at OX, the valve displacement is equal to 
OJ. This is at dead center and the valve is open the amount IJ, 
for it has moved this distance more than the outside lap. There¬ 
fore IJ is the lead for this end. 

Now on the other valve circle, draw the arc PQ with the 
inside lap inch) as a radius. It will cut the valve circle at 
P and Q. When the valve displacement is equal to OQ, the 
f exhaust port has just closed and the engine is at compression. In 
the same way OP is the valve displacement at release when the 
port begins to open. • OQD represents the crank position at com¬ 
pression and OPB the crank position at release. 

The results then are as follows : 

Given: 

Valve travel = XY = 3 inches. 

Angular advance = angle MOE = 35°. 

Inside lap = OP = % inch. 

Crank angle at cut-off = angle XOC 115° 

Found: 

Outside lap = OK = % inch. 

Angle of lead = XOA = 5°. 

Linear lead = IJ = ^ inch. 

Max. port opening for admission = HE = % inch. 

Crank angle at compression = XOD = 185° 

Crank angle at release = XOB = 65° 

Max. port opening for exhaust = FN = % inch. 

Fig. 26 is drawn full size, and all of these measurements may 
readily be verified. This figure is drawn for the head end only. 
If the crank angle at cut-off is the same on both ends, the Zeuner’s 
diagram for the crank end will be exactly like Fig. 26. 

ANOTHER PROBLEM. 

Given: 

The valve travel = 3 inches. 

The lead angle = 6°. 

Crank angle at cut-off' = 70°. 

Crank angle at compression = 75°. 

To Find: " : 

Angular advance. 

Laps. 

Linear lead. 

Crank angle at release. 

As before, let XY represent the valve travel = 3 inches and 
draw OM and the circle XMYF. See Fig. 27. Lay off the lead 



VALVE GEARS 


25 

angle XOA = 6°. Then OA represents the crank position at 
admission. Next lay off the crank angle XOC, the angle at cut¬ 
off 70°. Bisect the angle COA by the line OE and on OE draw 
the valve circle. Angle MOE - = the angular advance. The valve 
circle will cut the crank lines OC and OA at Kand V respectively. 
If the work has been carefully done, OK will be exactly equal to 
OV and will represent the outside lap. The lead is IJ as before. 
Draw OD at position of the crank at compression so that angle 
XOD = 75°. Continue OE to cut the eccentric circle at F. On 



OF draw the second valve circle. It will cut OD at Q, and OQ 
will represent the inside lap. Draw the lap circle OP, and the 
crank position OPB. This will be the crank position at release. 

The angular advance in this problem is large and all the 
events of the stroke are early. Compression and release are excess¬ 
ively early and the outside lap is unusually large. In the previ¬ 
ous problem, with cut-off at about two-thirds stroke, the results 
were nearly normal. Cut-off with the plain slide valve, earlier 
than half stroke cannot be had without sacrificing the steam dis¬ 
tribution on the other events. 









26 


VALVE GEARS 


To sum up we have 

^iven: 

Valve travel = XY = 3 inches. 

Lead angle = XOA = 6°. 

Crank angle at cut-off = XOO = 70°. 

Crank angle at compression = XOD = 75°. 


To find: 

Angular advance 
Outside lap 
Lead 
Inside lap 

Crank angle at release 


= MOE = 58°. 

= OV = 1 & inches. 
= IJ = & inch. 

= OQ = 3 r 2 inch. 

= XOB = 130°. 


Suppose in this last problem the cut-off had been given at 
half stroke instead of having the crank angle given, and that the 
compression had been given in the same way. "We should, of 
course, need to know the ratio of length of connecting rod to 
crank. Let this be given as 4, that is, the connecting rod is four . 
times the length of the crank. 

In Fig. 28 let XY represent the valve travel. Extend XY 
to the left to the point Z, and make OZ equal to four times OX. 
With Z as a center and OZ as a radius, strike an arc OC that will 
cut the eccentric circle at C; then draw OC, which will represent 
the crank when the piston is at half-stroke, which is assumed to 
be the point of cut-off. 

To find the crank angle at compression, lay off YII equal 
to .8 of the distance YX. From FL lay off FLW = OZ = four times 
OX. From W as a center with a radius WH, draw an arc cut¬ 
ting the eccentric circle at D. Draw OD, which will represent 
the position of the crank at compression. 

The student is advised to read over again pages 13 to 14 if this expla¬ 
nation of finding the crank angle does not seem perfectly clear. 

ANOTHER PROBLEM. 

Given: 

Cut off at .6 stroke. 

Lead = inch. 

Maximum port opening = % inch. 

Ratio of crank to connecting rod = 4. 

To find : 

The eccentricity. 

Lead angle. 

Angular advance. 

Laps. 




VALVE GEARS 


27 


In Fig. 29 assume an eccentricity that will if possible be a 
little too large. Let us take for trial 2J inches and draw XY 
equal to twice the assumed eccentricity equal to 4^ inches. Lay 
off XC' equal to .6 of XY, 
and with a radius equal to 
four times OX draw the arc 
C'C as already explained. 

Then draw OC, which 
will represent the position of 
the crank at cut-off, and 
XOC will be the crank-angle 
at cut-off. Assume a lead 
angle of about 7° and draw 
OA, which, if this assump¬ 
tion be true, will represent 
the crank-angleatadmission. 

Bisect the angle CO A by the 
line OE, and on OE draw 
the valve circle. Draw the 
lap circle VXK. With this 
assumed eccentricity we find 
a maximum port opening of 
NE = .75 inch, which is 
larger than the conditions of 
the problem demand. We 
may then form a proportion, 
namely: 

The actual port opening de¬ 
sired : the port opening with the 
assumed eccentricity:: probable 
eccentricity: assumed eccentric¬ 
ity. 

Substituting the figures 
we have .5 : .75 : : x : 2J 
x — the probable eccentric¬ 
ity; equals 1.42 inches. 

Now draw on OE, a new 
valve circle (dotted) with a diameter equal to the required eccen¬ 
tricity of 1.42 inches. See Fig. 29 a. It will cut the crank line 







28 


VALVE GEAKS 


OC at K', and OK' will be the new outside lap and I'J' will be 
the new lead (assuming the lead angle to be 7°). This lead I'J' is 
J inch, while the required lead is only inch. Now decrease the 
angular advance enough to correct one half of this difference, by 
drawing a new lap circle J"K" of inch greater radius. This 
will make the valve circle cut OC at K", so that OK" will now be 



remain the same. This reduces the port opening by the amount 
IIH , so that the maximum opening is only .40 inch. J3y increas¬ 
ing the eccentricity this port opening may be increased. 
















VALVE GEARS 


29 



x = 1.48, the true eccentricity. 

Now draw the valve circle on OE' with a diameter of 1.48 ' 

inches. It will cut OC in K'" and OX in J'". The lap will be 
OK"' = .97 inch, the lead will be T" J'" = T l „ inch, the angular 
advance will he MOE' and the eccentricity ON'. 

To sum up we have 
Given: 

The cut-ofT = .6 stroke. 

The lead = T V i uch 

Max. port opening = % inch. 


Fig. 29a. Essentials of Fig. 29 to larger scale. 












30 


VALVE GEARS 


Obtained all of the above conditions together with: 

Lap = .97 inch. 

Lead angle = XOA'° 

Angular advance = MOE' ° 

Compression, release and inside laps are found as in the pre¬ 
vious problems. 

There are of course all sorts of combinations that would make 
up different problems, but they can all be solved in the same gen¬ 
eral way, as they are modifications of the above. 


DESIGN OF THE SLIDE VALVE. 


In designing a slide valve some of the variables are assumed 
and the others are found by means of diagrams as we have already 
seen. These diagrams show only the dimensions of the inside and 
outside laps and travel of valve; the other dimensions of the valve 
and seat must be calculated. 

Area of Steam Pipe. Pipes that supply the steam chest 
should be large enough to prevent an excessive loss of pressure due 
to friction. If the pipes are long they should be of such size that 
the mean velocity of steam in them does not exceed 100 feet per 
second or 6,000 feet per minute. For this calculation it is usual 
to assume steam admitted to the cylinder during the whole stroke. 

For example. Suppose an engine is 10" X 18", and makes 
180 revolutions per minute. What is the diameter of the steam 
pipe ? 

The piston displacement or volume of the cylinder is : 

^ X l = x 18 = 1413.72 cubic inches. 


4 

1413.72 
1728 ' 


= .818 cubic feet. 


If the engine makes 180 revolutions it would use 2 X 180 X 
.818 — 294.48 cubic feet per minute. 

294.48 

The area would be = = .04908 sq. ft. = 7.0675 sq. in. 

The diameter corresponding to 7.0675 square inches is 3 
inches. 

A three-inch pipe would be large enough, especially if the 
engine cut off early in the stroke. 







VALVE GEARS 


31 


For a very large engine cutting off early, the allowable veloc¬ 
ity may be taken as 8,000 feet per minute instead of 6,000 feet. 

Width of Steam Port. The port opening at admission should 
give nearly as great an area as the steam pipe in order to prevent 
loss of pressure due to wire-drawing, but the actual width- of the 
port should be great enough for the free exhaust of steam. It 
is well to have the steam port a little larger than the area of the 
steam pipe, then with a port opening of .6 to .9 of the port area 
for admission and full port opening at exhaust, satisfactory condi¬ 
tions will result. 

The length of the ports is usually made about .8 the diameter oi 
the cylinder. Then in the 10". X 18" engine the steam ports would 
be 8 inches long. If the area for admitting steam is 8.0675 square 
inches and the length of port is 8 inches, the width will be 
7 0675 

= ——g —- — .8834 inch, or about J inch. 

The width of port opening would be about .9 X .8834 = .79506 
inch or about -i~| inch. 

Width of Exhaust Port. When the slide valve is at its 
maximum displacement, the valve overlapping the exhaust port 
as shown in Fig. 7 reduces the area more or less. In designing 
the valve, the exhaust port should be of such a width that the 
maximum displacement of the valve does not reduce the area of 
the exhaust port to less than the area of the steam port. It is not 
advisable to make the exhaust port too large for this increases 
the size of the valve and thus causes excessive friction. 

The height of the exhaust cavity should never be less than the 
width of the steam port, and may be made much higher to advantage. 

Width of Bridge. The bridge must be of sufficient width so 
that outside edges of the valve cannot uncover the exhaust port. 
The width of the steam port plus the width of the outside lap plus 
the width of the bridge must be greater than the maximum dis¬ 
placement. 

The width of the bridges should be not less than the thickness 
of the cylinder w r all in order to make a good casting. 

The Point of Cut=off. In the study of Indicators, it was shown 
that if the point of cut-off is early, the other events are not good. 
If a plain slide valve is used with an automatic cut-off, the cut-off 




32 


VALVE GEARS 


is hastened either by changing the eccentricity or by changing 
the angular advance. Either of these methods will accomplish the 
result at the expense of the compression which consequently will 
be earlier and excessive. Except for locomotives and high-speed 
engines, where compression is an advantage, the plain slide valve 
is not arranged to cut-off earlier than g or § stroke. If an earlier 
cut-off is desired, large outside laps are necessary. The cut-offs 
may be equalized by giving the head end a greater lap than the 
crank end, but this will cause an inequality of lead. 

Lead. The lead of stationary engines varies from zero to § 
inch according to the style of engine. An engine having high 
compression that compresses the steam nearly to boiler pressure, 
will give good results with little or no lead. If the ports are small, 
and the clearance large, there should be considerable lead in order 
to insure full initial pressure on the piston at the beginning of the 
stroke. Yalves that open slowly require more lead than quick¬ 
acting valves. 

Let us design and lay out the valve and valve seat for the fol¬ 
lowing engine: 

Diameter of cylinder = 10 inches. 

Stroke = 18 inches. 

Revolutions = 180 per minute. 

Lead angle = 3°. 

Cut-off to be equal at both ends and to take place at .75 stroke. 

Max. port opening = .9 area of steam pipe. 

Compression to be .85 of the stroke at both ends. 

Length of connecting rod = 3 feet. 

The piston displacement, or cylinder volume, will be 
3 1416 X10 2 

—--X18 = 1413.7 cubic inches or .818 cubic feet. If the 

4 

engine makes 180 revolutions, it will use 2 X 180 X .818 = 294.48 

294.48 

cubic feet of steam per minute. Steam pipe area = = .0491 

square feet = 7.07 square inches. 

This 7.07 square inches would also be the least possible area 
of the steam ports. If the length of port is made .8 the diam- 

7.07 

eter of cylinder, the width will be -g— = .88 inches or about g 

inch. The width of maximum port opening will be .9 X .88 = .792 
or nearly ] •* inch. 





VALVE GEARS 


33 


It will be necessary to draw a separate valve circle for each 
end of the cylinder. First consider the head end. 

The valve travel not being known, we shall lay off XY on an 
assumption of (1 inches travel and draw the eccentric circle as 
shown in Fig. 30. Lay off the lead angle' XOA == 3°. Lay off 
XC' = .75 of the assumed valve travel = 44 inches. Draw the 
arc CC' as previously explained and draw OC which will be the 
crank angle at cut-off. The radius of the arc C'C will be equal to 



4 times the radius of the eccentric circle, or 12 inches, because 
the connecting rod is 4 times the length of the crank. Bisect 
the angle AOC by the line OE, and on OE draw the valve circle. 
OV = OK is then the outside lap, with these assumed condi- 
















34 


VALVE GEARS 


tions. Draw tlie lap circle; then EN will he the maxi mum port 
opening. EN = 1 T 7 5 inches, while j j inch is all that is necessary. 
The assumed eccentricity is 3 inches, therefore the probable eccen¬ 
tricity — x : 3 : : -tf : ly 7 .. 'x — l j J- inches. 

Now draw a new eccentric circle with a radius of 1} J inches 
and a new valve circle with OE' = 1} J inches as a diameter. OK' 
is now the outside lap and the maximum port opening is equal to 
E'N', which from actual measurement is found to be } J inch. The 
outside lap = OK' = OV' = JJ inch and the lead is IJ = JV inch. 

Produce EG to F and draw another valve circle. We shall 
use this valve circle to determine the outside laps and lead for the 
crank end of the cylinder. Since the cut-off is to be .75 of the 
stroke, we may lay off OTP — OC', and with a radius of 12 inches 



draw the arc HIT. Then, as already explained, OH will be the 
crank angle at cut-off on the return stroke. OB will be the out¬ 
side lap = -JJ- inch. Draw the lap circle intersecting the valve 
circle at D. Then ODA' is the crank angle at admission on the 
return stroke and LM — | inch is the lead on the crank end of the 
cylinder. The maximum port opening will always be greater at the 
crank end than at the head end because the crank end lap is less 
in order to get the equal cut-off. If the laps were equal, of course 
the port openings would be equal. 

Now lay off YG' = .85 of XY and find the crank position 
OG. This is the compression on the head end of the cylinder and 
gives an inside lap on this end of inch, which is equal to OP. 




















VALVE LEAKS 


35 


Draw the lap circle PQ, which allows us to draw through Q the 
crank line OR, which is the release on the forward stroke. 

Lay off XS' = YG' = .85 of XY, and construct the crank 
line OS, which is the crank position at the crank end compression. 
OS intersects the valve circle at T, which gives OT — inch = 
inside lap on the crank end. Draw this lap circle, which will 
intersect the valve circle at U. This enables us to draw OUW, the 
crank angle at release, on the return stroke. 

From the data determined by means of these diagrams fhe 
valve may now be laid out. For convenience let us tabulate the 
results obtained as follows: 


Data. 

Head End. 

Crank End. 

Cut-off, per cent of Stroke 

75 

75 

Outside Lap 

2 7" 

1 9" 

inr 

Inside Lap 

A” 


Lead 

A 

i” 

Port Opening 

JL3" 

ii" 

1 i7 

Width of Port 

r 

7" 

■S’ 


Fig. 31 shows this valve in section. Let us begin at the end 
having the largest inside lap, or in this case at the crank end. 
Lay out the steam port | inch wide, and the crank-end outside lap 
= inch. The bridge will be, say, g inch wide. From the 
inner edge of the steam port, lay off the crank-end inside lap = 
j'tj inch. When the valve moves to the left, the point E' will 
travel lLJ inches, a distance equal to the eccentricity, and in this 
position of extreme displacement the exhaust port EF must be open 
an amount at least equal ^to the steam port, g inch. Therefore we 
layoff EF equal to +‘ g" = 2^". The inside lap overlaps the 
bridge nearly g inch, so that we shall have to make the exhaust 
port opening equal to 2g inches. Lay off § inch again for* the 
bridge and measure back inch, equal to the head-end inside 
lap. The port is g inch wide, and the head-end inside lap of -JJ 
inch completes the outline of the valve seat. 

VALVE SETTING. 

The principles of valve diagrams are useful in setting valves 
as well as in designing them. The valve is usually set as accu¬ 
rately as possible, and then, after indicator cards have been taken, 




VALVE GEARS 


36 


the final adjustment can be made to correct slight irregularities. 

The slide valve is so designed that the laps cannot be altered 
without considerable labor, and the radius of the eccentric, which 
determines the travel of the valve, is usually fixed. The adjust¬ 
able parts are commonly the length of the valve spindle and the 
angular advance of the eccentric. 

By lengthening or shortening the valve spindle, the valve is 
made to travel an equal distance each side of the mid-position. 
Moving the eccentric on the shaft makes the action of the valve 
earlier or later as the angular advance is increased or decreased. 

To Put the Engine on the Center. It is usual to put the 
engine on center before setting the valve. First put the engine 
in a position where the piston has nearly completed the outward 



stroke, and make a mark M on the guide opposite the corner of 
the crosshead or at some convenient place. Also make a mark, 
with a center punch, on the frame of the engine near the crank 
disc or on the floor. With this punch mark Pas a center, describe 
an arc C on the wheel rim, with a tram. A tram is a steel rod 
with its ends bent at right angles and sharpened. 

Turn the engine past the center until the mark on the guide 
again corresponds with the corner of the crosshead, and make 
another mark D on the wheel with the tram, keeping the same 
center. With the center of the pulley or crank disc as a center, 
describe an arc CD on the rim, which intersects the two arcs drawn 
with the tram. Bisect the arc CD on the rim, included between 
the two short arcs, and turn the engine until the new point E is at 



























VALVE GEARS 


37 


?i distance from the point on the frame equal to the length of the 
tram, in which position the engine will be on the center. 

The engine should always be moved in the direction in which 
it is to run so that the lost motion of the wrist pin and crank pin 
will be taken up the right way. In case the engine has been 
moved too far at any time, it should be turned back beyond the 
desired point and brought up to that point while the engine is 
moving the right way. 

To Set the Valve with Equal Lead. Set the engine on the 
dead point and give the eccentric the proper angular advance. 
Adjust the length of the valve spindle to give the proper lead for 
that end. Now place the engine on the other dead point and 
measure the lead at that end. If the leads are unequal, correct 
half the error by changing the length of the valve spindle and 
the other half by altering the angular advance. In case the valve 
gear has a rocker, the length of the spindle should be such that 
the rocker will move as designed. The angular advance should 
not be changed, but the equal lead should be obtained by means 
of the valve spindle or the eccentric rod. 

Second Method. In case it is difficult to turn an engine the 
following method may be used. First loosen the eccentric on 
the shaft and turn it around until it gives maximum port opening 
first at one end and then at the other. If the maximum port 
openings are not equal, make them so by changing the length of 
the valve spindle by half the difference. When the above adjust¬ 
ment has been made, set the engine on dead center and give the 
valve the proper lead by turning the eccentric on the shaft. The 
angular advance is thus adjusted. 

To Set the Valve for Equal Cut=off. Place the engine on the 
dead point, give the eccentric the proper angular advance and 
the valve the proper lead. Move the engine forward until cut-off 
occurs, then measure the displacement of the crosshead from the 
beginning of the stroke. Continue moving the engine forward, 
until cut-off takes place on the return stroke and measure the dis¬ 
placement of the crosshead from the beginning of this stroke to 
this point. 

In case the cut-off is earlier at the crank than at the head-end, 
the valve spindle is too short. Adjust the length of the spindle 




38 


VALVE GEARS 


so that the inequality will be corrected. Now set the engine on 
the dead point again and give the valve the proper lead by means 
of the eccentric. By repeating the process, making slight changes, 
the desired result will be obtained. 

MODIFICATIONS OF THE SLIDE VALVE. 

The ordinary slide valve is suitable for small engines; but for 
large sizes some method must be employed to balance the steam 
pressure on the back of the valve. With large valves, such for 
instance as those of locomotives or large marine engines, a great 
force is exerted by the steam, and the valve is forced against its 
seat so hard that a large amount of power is necessary to move it. 
This excessive pressure causes the valve to wear badly and is a 
dead loss to the engine. The larger the valve, the greater this 
loss will be. 

Piston Valve. To prevent excessive pressure on the back of 
the valve, the piston valve is commonly used, especially in marine 
engines. This valve consists of two pistons, which cover and 
uncover the ports in precisely the same manner as the laps of the 
plain slide valve. These pistons are secured to the valve stem in 
an approved manner and are fitted with packing rings. 

The valve seat consists of two short cylinders or tubes 
accurately bored to fit the pistons of the valve. The port open¬ 
ings are not continuous as in the plain slide valve, but consist of 
many small openings, the bars of metal between these openings 
preventing the packing rings from springing out into the ports. 

Steam may be admitted to the middle of the steam chest and 
exhausted from the ends or vice versa. With the former method, 
the live steam is well separated from the exhaust, and the valve- 
rod stuffing box is exposed to exhaust steam only. This is a good 
arrangement for the high-pressure cylinder; if used for a cylinder 
in which there is a vacuum, air may l$ak into the exhaust space 
through the valve-rod stuffing box. With this arrangement the 
steam laps must be inside and the exhaust laps on the outside ends. 

The piston valve may be laid out and designed by means of 
the Zeuner diagram just as if it were a plain slide valve, and the 
action is the same except that it is balanced so far as the steam 




VALVE GEARS 


39 


pressure is concerned; the power to drive it being only that neces¬ 
sary to overcome the friction due to the spring rings. 

Fig. 33 shows a section of the piston valve and the high- 
pressure cylinder for one of the engines of the V. S. S. “Massa¬ 
chusetts,” This valve consists of two pistons connected by a 
sleeve through which the valve rod passes. This valve rod is pro¬ 
longed to a small balancing piston, placed directly over the main 



valve. The upper end of the balancing cylinder does not admit 
steam, so that the steam pressure below the balancing piston will 
practically carry the weight of the piston valve, thus relieving the 
valve gear and making the balance more nearly complete. 

DoubIe=Ported Valve. Sometimes it is impossible to get 
sufficient port opening for engines of large diameter and short 
stroke, especially those having a plain slide valve with short travel. 





























































































40 


VALVE GEAKS 


This difficulty may be overcome by means of the double-ported 
valve shown in Fig. 34. It is equivalent to two plain slide valves, 
each having its laps. The inner valve is similar to a plain slide 
valve except that there is communication between the exhaust 
space and the exhaust space of the outer valve. Each passage to 
the cylinder has two ports; a bridge separates the exhaust of the 
outer valve from the steam space of the inner valve, and the outer 
valve is made long enough to admit steam to the inner valve. 



This valve may be considered as equivalent to two equal slide 
valves of the same travel, each having one-half the total port 
opening. To admit the same amount of steam as a plain slide 
valve, the double-ported valve requires but half the valve travel; 
this is advantageous in high-speed engines. 



To balance the excessive steam pressure, the back of the valve 
is sometimes provided with a projecting ring wliich'is fitted to a 
similar ring within the top of the valve chest. These rings are 
planed true, and fit so that steam is prevented from acting on the 
back of the valve. The space inside the rings is sometimes placed 
communication with the condenser. 














































VALVE GEARS 


41 


The Trick Valve. The defect of the plain slide valve, due to 
the slowness in opening and closing, is largely remedied in the 
trick valve, which is so made that a double volume of steam enters 
during admission. Thus a quick and full opening of the port is 
obtained with a small valve travel. 

In Fig. 35 the valve is shown in mid-position. It is similar 
to a plain slide valve except that there is a passage PP through it. 
It has an outside lap O and an inside lap I. The seat is raised 
and has steam ports SS, bridges BB, and exhaust port E. If the 
valve moves to the right a distance equal to the outside lap plus 
the lead, it will be in the position shown in Fig. 36. Steam will 
be admitted at the extreme left edge of the valve just the same as 
though it were a plain slide valve; also, since steam surrounds the 
valve it will be admitted through the passage as shown in Fig. 36. 




If the lead is the same as for a plain slide valve, inch for 
instance, this valve would give double the port opening, that is J 
inch, when the valve was open a distance equal to the lead. 

Fig. 37 shows the valve when it is in its extreme position to 
the right and the port is full open to steam. 

Piston valves are also made with a passage similar to that of 
the trick valve for double admission. The valve used with the 
Annington and Sims engine is perhaps the best example. 

Balanced Valves. Since there is a wide difference between 
the pressure of admission and exhaust, there must always be a 
great pressure acting upon the valve, causing it to run hard and 
wear excessively. The greater the steam pressure, the lower the 
pressure at exhaust and the larger the valve, the greater this pres¬ 
sure will be. 













42 


VALVE GEARS 


Piston valves are commonly used on the high and intermedi¬ 
ate cylinders of triple-expansion engines, and if well made and 
fitted with spring rings, should not leak. Small piston valves are 
often made without packing rings; but even if they fit accurately 
when new, they soon become worn and cause trouble. 

The double-ported valve, the trick valve, and others often have 
some device for relieving the pressure, such as a bronze ring or 
cylinder, fastened to the back of the valve. This ring is pressed 
by springs against a finished surface of the valve chest cover, and 
the space thus enclosed by the ring may be connected to the 
exhaust. There are numerous devices for balancing valves, but 
they are usually more or less expensive and are liable to cause 
trouble from leakage. 

STEPHENSON LINK MOTION. 

One of the earliest, and at present one of the most common 
mechanisms for reversing engines, or changing the ratio of expan¬ 
sion, is the Stephenson link motion, shown in Fig. 38. This illus¬ 
tration is taken from the drawings of a recent battleship engine, 
and may be considered the typical arrangement of the Stephenson 
gear as applied to marine practice. 

The two eccentrics E and E', whose centers are at C and C, 
respectively, are shown in their relative positions when the crank 
OA is at dead center. The eccentric rods R and R' are connected 
by forked ends to the link pins H and G. The link consists of 
two curved bars bolted together in such a manner that they may 
slide by the link block N. On the link are three sets of trunions; 
the two outer ones, or link pins, are fitted into the forked end of 
the eccentric rods, and the middle one, known as the saddle pin, 
is fitted into the end of the drag links FM. 

The valve stem has, at its lower end, a pivoted block N, called 
the link block, provided with slotted sides through which the links 
can slide from right to left. The reverse shaft, or rock shaft, K, 
here shown in full gear “ forward,” may be turned until F moves 
over to B; in this position the link will be pushed across the link 
block, and the valve will get its motion from the rod R f instead 
of from R as before. The link in this position would be full gear 
“ astern.” 



VALVE GEARS 


43 



Fig. 38, 














































44 


VALVE GEARS 


♦ 

In all large engines, such as marine, the reverse shaft is 
turned by power, but in smaller engines, such as locomotives, the 
engineer can turn the shaft by means of a lever. 

When set full gear forward, as in Fig. 38, the valve,, admits 
steam to the crank end of the cylinder, and the crank revolves as 



shown by the ariow. As the crank turns, both eccentrics impart 
motion to the link, but the “go ahead” link; pin H approximately 
coincides with the link block, so that nearly all its up-and-down 
motion is transmitted to the valve stem, while the “ go astern ” 



eccentric exerts but little effect upon the link block. Moving the 
drag links over to the extreme right reverses all these conditions 
by bringing the other link pin under the link block. In this posi¬ 
tion, steam will be admitted to the other end of the cylinder, and 
the engine will run in the opposite direction. This will be clearly 
seen by referring to Fig. 38, 







VALVE GEARS 


45 


When at full gear, either forward or backing, the valve moves 
as if there were really but one eccentric, while at intermediate 
points its motion is the result of the combined influence of both 
eccentrics, one tending in a measure to counteract the other. The 
effect of this is to shorten the valve travel the same as if the valve 
were driven by a new eccentric having less throw than either of 
the other two. 



Decreasing the valve travel causes cut-off to occur earlier 
compression is earlier, release later, and the lead is reduced some¬ 
what. If every point of the link moved in the arc of a circle 
when the drag link is shifted, the lead would not alter; but, since 
the eccentric rods about which each end swings are centered at 
different points, C and C', this is impossible. 

Figs. 39 and 40 show the tw T o principal ways of arranging 
the eccentric rods of a Stephenson gear. The first is said to have 
“open rods”, the second “crossed rods”; referring to whether 
the rods are crossed or open when both the eccentrics face the link. 
It can easily be seen that when the eccentrics shown in Fig. 39 
have turned through 180° they will be in the position shown in 
Fig. 41, but this is the same arrangement as before and is “ open ” 
rods. The full lines show the'positions in full gear forward, while 






46 


VALVE GEARS 


the dotted lines indicate the positions in mid gear. With open 
rods it will be seen that when at full gear the link block is at G, 
and that if, without turning the crank, the link is shifted to mid 
gear, then the link block moves to <J, Fig. 39, and the valve must 
consequently be moved toward the right an amount equal to GJ, 
thereby increasing the lead on the crank end of the cylinder. 
With crossed rods, moving the link from full to mid gear moves 
the link block from G to J, Fig. 40, thus reducing the lead. It 
follows then that open rods give increasing lead from full toward 
mid gear, and that crossed rods give decreasing lead. With crossed 
rods there will be no lead when in mid gear. It will be apparent 
that the shorter the rods the greater this increase or decrease 
will be. 



Nearly all marine engines, and some English locomotives, 
have their link blocks carried directly on the valve rod. Ameri¬ 
can locomotives commonly use a rocker, one end of which carries 
the link block while the other moves the valve rod. This arrange¬ 
ment indicated in Fig. 42 makes it possible to place the valve and 
steam chest above the cylinder. The position of the crank for the 
same valve position is just opposite that shown in Fig. 39 because 
the rocker reverses the valve motion; this gives an arrangement of 
crank and eccentrics that is identical with that indicated in Fig. 
41 and the rods, although apparently crossed, are in reality of the 
open rod arrangement, giving increasing lead toward mid gear. 
A rod from the bell-crank lever on the reverse shaft E, leads back 
to the engineer’s cab and connects with the reverse lever. This 
ever moves over a notched arc, and may be held by a latch in any 











VALVE GEARS 


47 


one of the notches, thus setting the link in any position from mid 
gear to full gear, either forward or back. 

The Stephenson link is designed to give equal lead at both 
ends of the cylinder; but to accomplish this, the radius of the link 
arc (that is an imaginary line in the center of the slot) must be 
equal to the distance from the center of this slot to the center of 
the eccentric. In Fig. 38 the radius of the link arc is equal to 
CH and C'G. 

Exact quality of lead is not essential, and the radius of the 
link arc is sometimes made greater or less than stated above in 
order to aid in equalizing the cut-off; but the change should never 
be great enough to affect the leads. 

Stephenson originally intended to use the link, simply as a 
reversing gear, but soon found, however, that at intermediate 
points between the two positions of full gear, it would serve very 
well as a means of varying the expansion and cut-off. Very soon 
the link came to be used not only on locomotives and marine 
engines, but on stationary engines as well, in connection with the 
reverse shaft which was under the control of the governor. The 
mechanism proved to be too heavy to be, easily moved by a gov¬ 
ernor and it has gradually fallen into disuse on stationary engines 
excepting as a means of reversing. 

In marine practice, the variable expansion feature is of little 
value, for marine engines run under a steady load and the link is 
set either at full gear or at some fixed cut-off. For locomotives, 
however, the variable expansion is nearly as important as reversing. 
Locomotives are generally started at full gear, admitting steam for 
nearly the entire stroke, and then exhausting it at relatively high 
pressure. This wasteful use of steam is necessary to furnish the 
power needed in starting a train. After the train is under way, 
less power is required per stroke, and the link is gradually moved 
toward mid gear, or “notched up” by the engineer, thus hasten¬ 
ing the cut-off; the expansion is increased and the power is reduced 
in proportion to the load. 

As the cut-off is changed, it is desirable to maintain an 
approximately equal cut-off at each end of the cylinder; this can 
be secured in the Stephenson gear by properly locating the saddle 
pin and the reverse shaft. When used without a rocker, as in 



48 


VALVE GEARS 


Fig. 38, the saddle pin should be on the arc of the link or slightly 
ahead of it. When used with a rocker, the saddle pin should be 
behind the link arcs, and to give symmetrical action for forward 
and backward running, it should be opposite the middle of the arc, 
that is, equally distant from each link pin. 

The Stephenson link cannot be designed directly from the 
Zeuner diagram, but a systematic investigation can be made by 
using a wooden model of the proposed link. This can be mounted 
on a drawing board, and the effect of changing the position of 
pins and the proportions of rods and levers can be determined 
without difficulty. By a system of trials a combination can be 
found best suited to obtain the desired results. Moreover, a 
model makes it possible to measure directly the slip of the link 
block along the link. This slip should be kept as small as possi¬ 
ble to prevent rapid wear. It can be controlled to some extent by 
properly locating the link pins, by avoiding too short a link, and 
by choosing a favorable position for the reverse shaft. 

The Gooch Link. Another form of link motion, known as 
the Gooch Link, is illustrated in Fig. 43. It has been extensively 
used on European locomotives, although it is gradually being 
replaced by a type of valve gear known as the Walschaert, which 
will be described later. 

The Gooch link has its concave side turned toward the valve 
instead of toward the eccentric. The radius of curvature of the 
link is equal to AB, the length of the radius rod. The link is 
stationary and the link block slides in the link. The engine is 
reversed by means of the bell-crank lever on the reverse shaft E 
which shifts the link block instead of the link, as is the case with 
the Stephenson. The link is suspended from its saddle pin M, 
which is connected by a rod to the fixed center F, so that the link 
can move forward and back as the eccentricity is changed, or it 
can pivot about its saddle pin as the eccentrics revolve. 

Since the radius of the link arc is equal to AB, it is apparent 
that the block can be moved from one end of the link to the other, 
that is, from full gear “ forward ” to full gear “ back ” without 
moving the point A, which is on the end of the valve rod. The 
lead then is constant for all positions of the block, and the distri¬ 
bution of steam for locomotives is slightly preferable to that 



VALVE GEARS 


49 


obtained by the Stephenson; but the gear is more complicated and 
requires nearly double the distance between shaft and valve stem. 

The variable lead is perhaps a slight advantage to the loco¬ 
motive, which is a slow-speed engine in starting, thus requiring 
but little lead. As the speed increases, and the link is “ notched 
up”, the lead is increased as the cut-off is shortened, and at high 
speed we have a large lead. With the Gooch link, the lead can be 
set for the average running speed, and although a little too great 
for good work at slow speed, it is a matter of small consequence, 
because the engine runs at slow speed but a very small fraction of 
the time it is in service, and the loss due to large lead at slow 
speed is ok no consequence whatever in a day’s run. 

Several other link motions have been used; but at the present 
time probably more Stephenson link motions are used than all 



the other forms of reversing gear combined, and when a “ link 
motion ” is mentioned, the Stephenson is usually meant unless 
otherwise specified. 

RADIAL VALVE GEARS. 

In general, it would be desirable to have precisely similar 
steam distribution at each end of the cylinder, and it would often 
be of great advantage with an expansion gear like the Stephenson, 
if the cut-off could be shortened without changing any other event 
of the stroke. A Stephenson gear can be made to maintain equality 
of lead for both ends of the cylinder as the cut-off is shortened, 
but we have seen that in so doing, the lead of both ends is either 












50 


VALVE GEARS 


increased or diminished according as the link is arranged with 
“ open rods ” or “crossed rods’ 5 . Moreover, the compression is 
hastened by bringing the link to mid-gear, all of which in many 
instances is undesirable. 

This disadvantage of the Stephenson link motion lead to the 
design of the so-called u Radial Valve Gears ”, many of which are 
so complicated as to be impracticable, but all of which obtain a 
fairly uniform distribution of steam. 



Hackworth Gear. The essential features of the llackworth 
Gear are indicated in outline in Eig. 44. In this figure, S is the 
center of the shaft, and the eccentric E is set 180° from the crank 
SH. At the right-hand end of the eccentric rod EA, is pivoted a 
block which slides in a straight, slotted guide. The guide remains 
stationary while the engine is running, but can be turned on its 





























VALVE GEARS 


51 


axis P, to reverse the engine or change the cut-off. P is a pivot, 
located on the horizontal through S in such a position that 
DP = EA. If these two distances are equal, A will coincide 
tvith P when the crank is at either dead point and the slotted 
guide may be turned from “ full gear forward ”, as shown in the 
figure, through the horizontal position to “ full gear backing ”, as 



shown by the line BL, without moving the valve. Therefore the 
leads are constant for all positions of the guide. The valve rod 
running upward from C, connects with the valve stem which it 
moves in a straight line. The valve stem is made just long 
enough to equalize both leads, and if the point C has been properly 
chosen, the two cut-offs will be very nearly equal for all grades of 
the gear. 




























52 


VALVE GEARS 


A somewhat better valve action is obtained by slightly curv¬ 
ing the slotted guide, with its convex side downward. This gear 
is sometimes used on marine engines and on small stationary 
engines. 

Marshall Gear. The most objectionable feature of the Hack- 
worth gear, is the slotted guide, for the sliding of the block causes 
considerable friction and wear. The Marshall gear, shown in 
outline in Fig. 45, is designed to obviate this feature. The point 
A moves in the desired path by swinging on the rod FA about F 
as a center. While the engine is running, the lever FP remains 
stationary, but can be turned on its axis P to reverse the engine, 
or change the cut-off. The pivot P, is located precisely as in the 
Hackworth gear, and the lever FP can be turned from u full gear 
forward”, as shown in the figure, to (i full gear backing ”, as shown 
by the line BP, intermediate positions give different cut-offs as 
with the Hackworth gear. Since FA is made equal to FP, the- 
point A will always sw T ing through P, no matter where F may 
be, and will coincide with P, when the engine is on dead center. 
The leads therefore will remain constant, as in the preceding case. 

The Marshall gear is sometimes made with C at the right of 
A on a prolongation of the line EA. In this case if the same 
kind of valve is to be used, the eccentric E must move with the 
crank instead of 180° from it. The Marshall gear is frequently 
used on marine engines, the one eccentric being simpler than the 
two required by the Stephenson. 

Joy Gear. Perhaps the most widely known, and certainly 
one of the best radial gears is the Joy, outlined in Fig. 40. It 
is frequently used on marine engines and on some English loco¬ 
motives. No eccentrics are used, the valve motion being taken 
from C, a point on the connecting rod. H is a fixed pivot sup¬ 
ported on the cylinder casting. The lever ED has a block pivoted 
at A, w T hick slides back and forth in a curved slotted guide. The 
guide and the lever PF are fastened to the reverse shaft P, and 
by means of a reverse rod leading off from F, can be turned from 
full gear forward, as shown, to full gear backing when the pin F 
moves over to B. Motion is transmitted to the valve stem by 
means of the radius rod EG. The proportions are such that when 
the crank is on either dead point, the pivot of block A coincides 



VALVE GEARS 


53 


with P, so that the curved guide may then be set in any position 
without moving the valve; therefore the leads are constant. This 
gear gives a rapid motion to the valve when opening and closing 
and a more nearly constant compression than the Stephenson gear, 
and the cut-off can be made very nearly equal for all grades of 
the gear. Its many joints cause wear and its position near the 
crosshead, makes a careful inspection of the crosshead and piston 
exceedingly difficult while the engine is running. 



Fig. 46. 

Walschaert Gear. This radial valve gear, although seldom 
seen in the United States, is the valve mechanism most commonly 
used on locomotives built on the continent of Europe. Like all 
other radial gears, it gives constant lead, and a distribution of 
steam very nearly alike for each end of the cylinder. In this 
respect it is superior to the Stephenson link, and gives without 
doubt better economy, but its mechanical construction is compli¬ 
cated, and not well adapted to the American type of locomotive. 
Fig. 47 illustrates this type of gear. S is the cejytef of the driver 
axle. The crank pin K has forged on its center end an arm KE, 
on which the pin E is fixed. This arm lies parallel to the plane 
of the driving wheels, and being fixed to the crank pin, turns with 
the wheel, allowing the connecting rod to pass between it and the 
driving wheels. In this manner the point E moves around S in a 
circle, and moves the rod Eli back and forth just as if it were an 
eccentric. It is so made that ES is perpendicular to the crank 
KS, and therefore the action of the pin E is equivalent to an eccen¬ 
tric with no angular advance. 













54 


VALVE GEAKS 


This arm reaching back from the outer end of the crank pin 
is one of the most objectionable features on the construction, and 
is sometimes replaced by the regular type of eccentric put on the 
shaft between the driving wheels. 

The eccentric rod EIT causes the box link HP to oscillate on 
fixed trunions P. This link has a groove curved to a radius equal 
to GD, the length of the radius rod. A block pivoted at G, on 
one end of the radius rod, is free to move up or down in this 



Fig. 47. 

groove. The valve derives its motion from C, a pivot on the float¬ 
ing lever CA. Point A receives motion from the crosshead; point 
D from the eccentric and the curved link ; and a combination of 
these two imparts motion to C (which can slide only along the 
dotted line). A bell-crank lever pivoted above the link shifts 
the mechanism from “ full gear forward ” when F is moved to B, 
thus raising G above the link pivot or saddle pin. 

ADJUSTABLE ECCENTRICS. 

The position of an eccentric for a plain slide valve is 90° plus 
the angular advance ahead of the crank, in the direction in which 
the engine is to turn. Thus A, Fig. 48, is correctly placed, rela¬ 
tive to the crank C, if the engine turns right-handed. For run¬ 
ning in the opposite direction, the position of the eccentric is at 
D. Some engines are provided with a reversing mechanism which 
causes the eccentric to shift from A to D, either along the arc 




















VALVE GEARS 


55 


ABD, or along the straight line AED. Such engines provide, 
not only for reversing, hut for changing the cut-off as well If 
the eccentric moves on the arc to OB, the angular advance is 



increased and all the events of the stroke are hastened as well as 
the cut-off, but the travel of the valve is not changed. 

Zeuner’s diagram, Fig. 49, is lettered to correspond with Fig. 
48, and shows the effect of changing the angular advance from 
FOA to FOB. If OK rep¬ 
resents the lap, the crank 
angle at cut-off will decrease 
from IIOK to HOL, and the 
lead will increase very much, 
viz., from GF to HF. If the 
eccentric is shifted on the 
straight line to E (Fig. 48), 
a different valve motion will 
result. The angular advance 
is increased as before, so that 
all the events are hastened, 
but the eccentricity is now 
only OE instead of OB and 
the valve travel is conse¬ 
quently reduced. Zeuner’s diagram for this case, Fig. 50, shows a 
decrease in crank angle at cut-off from 10M to ION, and no 















56 


VALVE GEARS 


change in the lead IF. Let 
us consider the eccentric posi¬ 
tion OA, Fig. 48. In this 
position 01 represents the 
displacement of the valve 
from mid-position when the 
engine is on center. If the 
eccentric moves to OB, the 
displacement will he OM, 
which is greater, showing an 
increase in lead equal to IM, 
but if the position is OE in¬ 
stead of OB, the displace- 
Fig. 50. 7 r 

ment from mid-position will 

be OI as before. It is evident that the eccentric can move on the 
straight line from A to D, without changing the lead, while the 
decreased valve travel will restilt in an earlier cut-off. If the 


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//u 



p\ 



V 

/ 

/ 

/ 

\ / 

\ 1 
\ 1 

VV- 

/ 

nC? h 

\ I 

\ / / 

\ / / 

—/ 

1 / 

\ \ 

\\ 

\\ 

D 

/ 

/ 

/ 

/ 

/ / 

/ / 

/ / 

/ y 

* / 



shifting eccentric is to be used for -an automatic cut-off, as in 
the various types of fly-wheel governor, the curved path is not 




















VALVE GEAKS 


57 


desirable oil account of excessive lead at short cut-off, but if it is 
to be used only as a means of reversing, it is preferable to the 
straight line. 

All fly-wheel governors operate by shifting the eccentric, 
either to change the angular advance, the travel of the valve, or 
both. Fig. 51 illustrates the principle of a governor arranged to 
give decreasing lead, but as these mechanisms are described in 
the Steam Engine—Part I, under the head of governors, a further 
discussion will not be given here. 



The device shown in Fig. 52 is often used for reversing 
engines of small launches. The eccentric E is loose on the shaft 
between a fixed collar G, and a hand wheel II. A stud project¬ 
ing from the eccentric passes through a curved slot in the disc of 
the wheel, and can be clamped by a hand nut F. When running 
forward with the crank at C, 
the center of the eccentric is at 
A, and the nut clamped at F. 

To reverse, steam is shut off, 
and when the engine stops, the 
nut F is loosened, and then 
moved to B and clamped; or 
after F is loosened, the wheel, 
shaft, crank and propeller are 
turned over by hand until B 
strikes the stud at F, where it is 
clamped. The engine will then 
run astern. 

To study the application of 
the Zeuner diagram to this 
form of mechanism turn again 
to Fig. 51. If OA is the de¬ 
sired eccentricity for a normal 
position of the governor, the 
perpendicular distance of A 
from OF is made equal to the lap 01, plus the desired lead. Pivot 
D is then located equally distant from A and I. Zeuner’s dia¬ 
gram for this gear, drawn to an enlarged scale, is shown in fig. 5d. 
The angular advance F*OB is laid back toward OC. OB is the 

























58 


VALVE GEARS 


maximum eccentricity; 01, the lap or the desired least eccen¬ 
tricity. An arc, with proper radius, described through B and I 
shows the path of the eccentric. If the eccentric moves in to A, 
the crank angle at cut-off is decreased from COD to COE, and 
the lead decreased from FI to GI. A slight decrease in lead is 



not objectionable, since the speed is not allowed to increase more 
than two or three per cent; and further, as the lead increases, com¬ 
pression decreases, so that one influence helps to counteract the other. 
The decrease in maximum port opening from BH to AK is un¬ 
avoidable, but it is permissible, since it occurs only when the load 
decreases, and when less steam should be admitted to the cylinder. 

DOUBLE VALVE GEARS. 

It has been shown in the preceding discussion, that a plain 
slide valve under the control of a gear that gives a variable 
cut-off, such as a shifting eccentric or a link motion, will not 
give a satisfactory distribution of steam at short cut-off owing 
to execessive compression, variable lead, or early release. These 
difficulties are overcome in a measure by the use of the radial 
gear ; and also by the use of two valves instead of one. The main 







VALVE GEARS 


59 



valve controls admission, re¬ 
lease, and compression ; the 
other, called the cut-off valve, 
regulates the cut-off only, 
which may be changed with¬ 
out in any way affecting the 
other events of the stroke. 
This cut-off valve may be 
placed in a separate valve 
chest, or it may be placed on 
the back of the main valve. 

Meyer Valve. The most 
common form of double valve 
gear is the Meyer Valve, Fig. 
54. The cut-off valve is made 
in two parts and works on the 
back of the main valve. The 
two parts are connected to a 
valve spindle with a right- 
and left-hand thread, so that 
their positions may be altered 
by rotating the valve spindle. 

A swivel joint is usually 
fitted in the valve-spindle be¬ 
tween the steam chest and the 
head of the valve rod, and the 
valve spindle prolonged into 
a tail rod which projects 
through a stuffing box on the 
head of the steam chest. See 
Fig. 55. The end of this tail 
rod is square in section and 
reciprocates through a small 
hand wheel by means of which 
it can be rotated while the en¬ 
gine is running, whatever the 
position of the valve may be. 




































60 


VALVE GEARS 


Each valve is under the control of a separate eccentric. The 
eccentric moving the main valve is usually fixed, while the cut-off 
valve eccentric may be under the control of a governor. Since 
a slight compression is desired, the main valve is set to give late 
cut-off. This will give late release and late compression, and allow 
a wide range of cut-off for the cut-off valve. With this gear, lead, 
release, and compression are entirely independent of the ratio of 
expansion, and the cut-off is much sharper, because the cut-off 
valve, when closing the ports, is always moving in a direction 



opposite to that of the main valve. The valve may be designed 
by means of Zeuner’s diagrams. 

Design of a Meyer Valve. Let us design a Meyer Valve 
having an eccentricity of 2 inches. Let the eccentricity of the 
cut-off valve be 2^ inches and the relative travel of the cut-off 
valve in relation to the main valve be 3 inches. This will make 
the relative motion of the cut-off valve equivalent to the travel of 
a plain slide valve with an eccentricity of 1J inches. Let the out¬ 
side lap on the main valve he | inch, the lead inch, the com¬ 
pression 95 per cent of the stroke, and let the ratio of the length 
of the crank to connecting rod be six. 


























VALVE GEARS 


61 


In Fig. 56 draw XOY equal to 4 inches, the main valve 
travel. Lay off YD = 95 per cent of 4 = 3.8 inches, and with 
a radius of 12 inches, and the center on YX produced draw the arc 
DH.Iv. H.K.O is the crank position at compression. C.K.O, the 
crank position at cut-off, is found in a similar manner. Lay off 01 
equal to the lap plus the lead, and draw the valve circle for the 
main valve through I and O with a diameter equal to its eccen¬ 
tricity of 2 inches. To do this take a radius equal to 1 inch, and 
draw arcs from I and O as centers that shall intersect at B. B is 



tne center of the valve circle and OBE is the eccentricity, 2 inches. 
With E as a center, and with a radius equal to half the relative 
travel of the cut-off valve (in this case 1J inches), draw an arc. 
With O as a center and with a radius equal to inches, the 
eccentricity of the cut-off valve, draw another arc intersecting the 
first one at F. On OF as a diameter construct a valve circle. 
This valve will represent the absolute motion of the cut-off valve, 
independent of the motion of the main valve. This circle then 
will show the displacements of the cut-off valve from the center of 
the steam chest. With E as a center and with a radius equal to 
FO draw an arc, and with O as a center and with a radius equal 














62 


VALVE GEARS 


to EF draw another arc intersecting the first at G. On OG as a 
diameter construct a valve circle. This circle will then represent 
a travel of the cut-off valve moving on the main valve. That is, 
it will represent the displacements of the cut-off valve from the 
center of the main valve. This circle is not, properly speaking, 



a valve circle and OG is not an eccentricity, but simply represents 
the relative motion of the two valves. This can be proved by 
analytical geometry, but an inspection of the figure shows that 
this must be true. 

Draw the crank line OC at any position, cutting the valve 
circles at a and b and c. O a represents the absolute displace¬ 
ment of the cut-off valve, that is, from the center of the steam 
chest and O c represents the displacement of the main valve. The 













VALVE GEARS 


63 


relative displacement of the cut-off valve, that is, from the center 
of the main valve, will he the difference between O c and O a, since 
both valves are moving in the same direction. By careful meas¬ 
urement it will be found that Ob = O c — Oa, and any arc as Ob 
on the auxiliary circle O&G will correctly represent the displace¬ 
ment of the cut-off valve from the center of the main valve at the 
corresponding crank angle. 

Fig. 57 shows the crank angle at head-end compression H.K., 
and at crank-end compression C.K., the main valve circle, and the 
auxiliary circle which are transferred from Fig. 56. The con¬ 
struction lines and all lines not essential to the figure are omitted 
to avoid confusion. 

Lay off on Fig. 57, 01 equal to the outside lap £ inch and 
draw the head-end lap circle H.E.O. It will intersect the valve 
circle for the main valve at L and M. Through L draw the crank 
position at admission (head-end) H.A. and the crank position at 
cut-off through M. This gives the greatest possible cut-off. The 
cut-off valve may be set to give a much earlier cut-off than this, 
but of course a later setting would be of no avail for the port 
would be closed by the main valve at this angle. The crank line 
OMH cuts the auxiliary circle at FT, so that OH (1-iJ- inches) is 
the clearance of the cut-off valve. That is, the edge of the cut-off 
valve must be set lij- inches from the edge of the main valve port 
in order to cut-off at this crank angle. The full lines of Fig. 54 
show the cut-off valve placed in this position. 

The intersection of H.K.O with the lower valve circle, gives 
the inside lap at the head end of the cylinder. This line comes so 
nearly tangent to the valve circle that the intersection can be 
determined only by dropping a perpendicular to H.K.O. from E 1 . 
This cuts the circle at P and OP — inch equals the head-end 
inside lap, and II.E.I. represents the corresponding lap circle. 

The crank-end angle at compression is C.K. which cuts the 
upper valve circle at H', giving an inside lap for the crank end of 
OH' = -JJ inch. To make this intersection more apparent the 
perpendicular can be drawn from E as previously explained. 

Suppose that it was required that the minimum cut-off should 
be 15 per cent. Find the crank position at 15 per cent of the stroke 
in the same manner as the crank position was found at compression. 



64 


VALVE GEARS 


Produce this line through O until it cuts the auxiliary circle at S. 
Then OS = inch = the required lap for the cut-off valve in 
order to cut-off at 15 per cent of the stroke. The dotted lines in 
Fig. 54 show the cut-off valve drawn in this position. 

For a valve of this sort, the cylinder port should be 1J inches 
wide and the valve port 1 inch wide. Fig. 54 shows this valve laid 
out to scale, but as this process is in all respects similar to that 
described for laying out a plain slide valve, it will not be described 
in detail. 

DROP CUT=OFF GEARS. 

The ordinary slide valve controls eight different events of the 
stroke, that is, admission, cut-off, release, and compression for both 
ends of the cylinder. A change in the setting of a plain slide 
valve that affects any one event on the crank end, let us say, will 
also change to a greater or less degree every other event of the 
stroke, on the head end as well as on the crank end; so that in 
setting a slide valve the desired position for one event must 
usually be sacrificed in order to make the others less objectionable. 

In order to provide a better distribution of steam than is pos¬ 
sible with a single valve, some engines have four valves, two at 
each end of the cylinder. In horizontal engines, two are placed 
above the center line of cylinder and two below. The upper are 
for admission and cut-off, the lower for release and compres¬ 
sion. Since each valve controls but two events, a very satisfactory 
adjustment can be made and the extra complication and cost for 
large engines are more than overbalanced by the advantages 
gained, viz.: A very much better distribution of steam, short 
steam passages and small clearances, separate ports for the admis¬ 
sion of hot steam and the exhaust of the same steam after expan¬ 
sion when its temperature has fallen, and finally the possibility of 
opening and closing the ports very rapidly, thus preventing wire¬ 
drawing. The small clearances, short ports and separate admis¬ 
sion and exhaust materially reduce the cylinder condensation, and 
thus effect a large saving in the steam consumption. 

When four valves are used for high speeds, the motions of 
all must be positive, that is, they must be connected directly to 
some mechanism that either pushes or pulls them through their 
entire motion, but for speeds up to 100 revolutions or so a disen- 



VALVE GEARS 


65 


gaging mechanism may be used, and the valves may shut of them¬ 
selves, either by virtue of their weight or by means of springs or 
dashpots. The valve is usually opened by means of links or rods, 
moved by an eccentric, and at the proper point of cut-off the 
rod is disengaged from the valve which drops shut, hence the term 
“ drop cut-off 55 gears. 

ReynoIds=Corliss Gear. The most widely known drop cut¬ 
off gear is the Reynolds-Corliss, shown in Figs. 58 and 59; it is 
often referred to as the Reynolds hook-releasing qq ar. An eccen¬ 
tric on the main shaft gives an oscillating motion to a circular disc 



called the wrist plate, pivoted at the center of the cylinder. It 
transmits motion to each of the four valves through adjustable 
links known as steam rods or exhaust rods , according to whether 
they move the admission or exhaust valves. 

The valves which are shown in section in Fig. 60 oscillate on 
cylindrical seats, and the position of the rods is so determined that 
they give a rapid motion to the valve when opening or closing, and 
hold it nearly stationary when either opened or closed. 

The Reynolds hook is shown in detail in Fig. 59. The steam 
arm is keyed to the valve spindle which passes loosely through a 
bracket on which the bell-crank lever turns, and the spindle is 
packed to make a steam-tight joint where it enters the cylinder. 
Motion of the steam rod toward the right will turn the bell-crank 
lever and raise the hook stud . The hook (from which the gear 
derives its name) pivoted on this stud, has at one end a hard- 

































m 


VALVE GEARS 


(3 



05 

iO 


be 
























































































VALVE GEARS 


67 


ened steel die with sharp, square edges, and at the other end, 
a small steel block with a rounded face. As the hook rises, the 
hook die engages the stud die which is fastened to the steam arm , 
and one end of the steam arm is thus raised. This turns the 
valve in its seat and admits steam. As the hook continues to 
rise, its stud moves in an arc above the valve spindle, and the 
round-faced block at its left-hand end strikes the knock-off cam 
which causes the hook to turn about its stud and disen gaffe the 
hook die from the stud die. In raising the steam arm , the dash- 
pot rod also is raised and a partial vacuum is created in the dash- 
pot. As soon, therefore, as the dies become disengaged, the 
dashpot rod quickly drops under the force of this vacuum, thus 
turning the steam arm and closing the valve. The striking of the 
left-hand end of the hook against the knock-off cam determines 
the point of cut-off, by releasing the valve at that instant. 

This cam is a part of the knock-off lever to which the governor 
cam rod is fastened. Any action of the governor which would 
cause the cam rod to move toward the right would cause this 
knock-off lever to turn on its axis, the steam arm, and conse¬ 
quently lower the position of the knock-off cam. This would 
cause an earlier contact between the cam and end of hook, and 
consequently an earlier cut-off. By lengthening or shortening 
the governor cam rod , the point of cut-off can be adjusted to suit 
the engine load without changing the speed. 

There is a limit to this adjustment, for it can be shown that 
a Corliss gear operated by a single eccentric cannot be arranged to 
cut-off later than half stroke. Suppose the eccentric is set just 90° 
ahead of the crank. Then it will reach its extreme position just 
as the piston gets to half stroke. If by that time the hook which 
was rising and opening the admission valve, has not yet struck 
the knock-off cam , it cannot strike it at all, for any further motion 
will cause the hook to descend to its original position, that is its 
position at the beginning of the stroke; the hook will not disen¬ 
gage from the steam arm stud at all and the bell crank will 
return, closing the valve in the same manner in which it opened 
it. Cut-off will then take place near the end of the stroke, but it 
will not be the sharp cut-off produced by the sudden drop when 
the dies are disengaged. 




VALVE GEARS 


68 

If the eccentric were set less than 90° ahead of the crank, the 
cut-off could be arranged to occur later than half stroke, but this 
is decidedly impracticable, for with such a position of the eccen¬ 
tric the action of the valves at release and compression is spoiled. 
When it is necessary to cut-off later than half-stroke, as some¬ 
times happens on low-pressure cylinders of compound engines, it 
may be arranged for by means of two eccentrics, one set more 
than 90° ahead of the crank to operate the exhaust valves, and one 
less than 90° ahead to operate the admission valves. 

The safety cam shown in Fig. 
59 is an important part of a Cor¬ 
liss gear. If for any reason the 
engine governor should fail to 
act, due, for instance, to the 
breaking of its driving belt, the 
governor would drop to its low¬ 
est position, supply more steam 
to the engine and allow it to 
run away. The safety cam pre¬ 
vents this by moving so far to 
the right that it strikes the hook 
when it descends to pick up the 
steam arm. The hook is conse¬ 
quently turned toward the right 
and then lifted without engaging 
the stud die; the valve conse¬ 
quently remains closed and the engine stops. 

Brown Releasing Gear. In addition to the Reynolds hook , 
several other devices are in use for opening and releasing Corliss 
admission valves. Among them the Brown releasing gear shown 
in Fig. 61 may be noted. The steam rod and dashpot rod are 
arranged much the same as in the Reynolds gear. The governor 
cam rod operates a plate cam having a curved slot so shaped that it 
takes the place of both the knock-off and the safety cam of Fig. 59. 
The steam arm is keyed to the valve spindle and . carries at its 
lower end a steel die which is free to slip up and down a small 
amount. The part* of this gear corresponding to the Reynolds 
bell crank becomes a straight rocker pivoted at its middle; 



Fig. 60. 







































VALVE GEARS 


69 


and the part corresponding to the Reynolds hook has at one end 
a die which engages the die of the steam arm, and at its other 
end a roller running in.the curved cam slot. This hook is really 
a bell-crank lever with arms that are not in the same plane. The 
bearing; on which it turns is carried on the lower end of the 
rocker, and therefore is equivalent to a movable pivot similar to 
the hook stud of the Reynolds gear. 



Fig. 61. 


In the position shown the dies are engaged. Motion of the 
steam rod toward the right will move the lower end of the rocker 
toward the left, and consequently turn the valve spindle in a right- 
handed direction. This will open the valve and at the same time 
raise the dashpot rod. Meanwhile, the roller is moving toward 
the left in a circular part of the cam slot, the center of which is 
at the center of the valve spindle. This causes the steam arm and 
the bell-crank lever, which has the roller at one end, to move in 
such a way that there is no relative motion between them. As 
soon, however, as the roller comes to the point where it is forced 
to move out of this circular path and move farther from the valve 
spindle, both arms of the bell-crank lever are turned downward, 
























70 


VALVE GEARS 


the dies become disengaged, and the dashpot closes the valve. 
The slight up-and-down motion of the steam-arm die allows it to 
rise while the hook die passes underneath when returning to re¬ 
engage for the next stroke. The makers claim that this gear per¬ 
mits a much higher speed than is possible with other Corliss gears. 

Greene Gear. Another well-known drop cut-off gear is the 
Greene, shown in Fig. 62. The valves are of the gridiron type, 
sliding on horizontal seats, the admission valves parallel to, and 
the exhaust valves at right angles to the axis of the cylinder and 
just below it. AA are rock shafts turning in fixed bearings. 



BB are the admission valve stems. C is a slide bar, receiving a 
reciprocating motion from an eccentric. TT are tappets connected 
to the slide bar. They move to and fro with the slide bar and can 
also move independently up and down. They are made fast at 
their lower end to the gauge plate I) which slides through the 
guide E. The guide E is made fast to the governor rod F and 
through this means can be raised or lowered, thus regulating the 
height of the tappets. 

As the slide bar moves toward the right, the right-hand 
tappet is brought into contact with the toe of the rocker, causing 
it to turn on its bearings and move the rock lever and the valve 
stem B toward the right, thus opening the admission valve. Since 




































VALVE GEARS 


71 


the tappet moves in a horizontal direction while the toe of the 
rocker moves in an arc, it will be seen at once that they will soon 
become disengaged and release the valve which is at once closed by 
a dashpot not shown in the figure. If the governor raises the 
tappets, cut-off will be later. A nut at the bottom of the governor 
rod allows a proper adjustment of the guide and gauge plate. As 
the slide plate C moves tow T ard the right, the left-hand tappet comes 
in contact wfith the heel of the left-hand rocker, both being beveled, 
it rises in its socket allowing the tappet to pass under. It then 
falls by its own weight and is ready to engage the tappet on its 



Fig. 63. 


return and open the valve. In this gear the disengagement of the 
valve throws no load whatever on the governor which is a distinct 
advantage over the Corliss gear. The action of the exhaust valves 
is not shown in the cut. 

The Sulzer Gear is a drop cut-off widely used in Europe. 
The valves are of the poppet type, lifting straight from conical 
seats, so that there is no friction. They are usually placed verti¬ 
cally above and below the cylinder axis and are operated by eccen¬ 
trics from a shaft geared to the main shaft. The admission valves 
are lifted from their seats by suitable levers, then released by a 












































































72 


VALVE GEARS 


device equivalent in action to the Reynolds hook and are quickly 
closed by the action of springs. 

The exhaust valves of all drop cut-off gears are comparatively 
simple in their operation and both in opening and closing they are 
moved by the direct action of the exhaust rods. 

A common form of vacuum dashpot for closing admission 
valves is shown in Fig. 63. The rod coming down from the steam 
arm makes a ball-and-socket joint with the dashpot piston. The 
dashpot is often let down into the engine frame as shown. When 
lifted, the piston produces a partial vacuum underneath it so that 
it tends to drop quickly as soon as the valve gear is released. On 
some of the largest modern engines where the valves are very 
heavy, steam-loaded dashpots are used; that is, the dashpot piston 
has steam pressure 6n one side, and an air cushion on the other 
prevents it from striking the bottom of the dashpot. 

Corliss Valve Setting. The setting of a Corliss valve gear 

is a much longer process than 
the setting of a plain slide valve, 
but is nevertheless a compar¬ 
atively simple matter, for the 
various adjustments are nearly 
all independent of one another. 
In gears like that shown in Fig. 
58 the length of both the eccen¬ 
tric rod and carrier rod are 
unusually adjustable, and the 
former should be of such length 
that the carrier arm swings equai distances on each side of a verti¬ 
cal line through its pivot, and the carrier rod should be adjusted 
until the wrist plate oscillates symmetrically about a vertical line 
through its pivot Nearly all Corliss engines have one mark on 
the wrist plate hub and three on the wrist plate stand, as shown 
in Fig. 64, and the wrist plate should swing so that A, the mark 
it carries, moves from C to D, but not beyond either one. When 
A is in line with B, the wrist plate is in mid-position. The valves 
are then not in their exact mid-position, but it is customary to regard 
them as being in mid-position, an$ to speak of the lap as the amount 
the valve covers the port when the wrist plate is in mid-position. 
















VALVE GEARS 


73 


To set the valves, remove the bonnets or covers of the valve 
chambers on the side opposite the gear. The ends of the valves 
are circular, but inside their cross-section is as shown in Fig. 65. 
On the end, in line with the finished edge of the valve, and on 
the seat in line with the edge of the steam port, are marks as 
shown in Fig. 65. When these marks coincide, the valve is either 
just opening or just closing, and when in ahy other position, the 
amount of opening or the amount by which the port is closed is 
shown directly by the distance between the marks. Block the 
wrist plate in mid-position, hook up the admission valves and 
adjust the length of the steam rods by means of the right and left 
threads provided for the purpose, until the ports are covered by 
the amount of lap indicated in the following table opposite the 
given size of engine. 


Dia. of Cyl. 
in inches. 

12 

14 to 16 
18 to 22 
24 to 28 
30 to 36 
36 to 42 


Steam Lap 
in inches. 


IT 

S 


Exhaust Clearance 
in inches. 


TJ 

tV 

3 

TP 

1 


Next adjust the exhaust rods until the exhaust ports are open 
an amount equal to the clear¬ 
ance given in the above table. 

Set the engine on its f head- 
end dead point, hook the car¬ 
rier rod onto the wrist plate 
and in the direction in which 
the engine is to run, turn the 
eccentric enough to open the 
head-end admission valve by 
a proper amount of lead; 
then the eccentric will be 90° 
plus the angular advance 
ahead of the crank. The 

proper amount of lead will depend upon both the design of the gear 
and the speed at which the engine is to run ; and may vary from 
y y' for small engines to as much as or for large and higher- 



MARKS 


Fig. 65. 



















74 


VALVE GEAKS 


speed engines. When the proper amount of lead has been obtained, 
fasten the eccentric on the shaft by means of the set screw and 
make sure by trial that the wrist plate moves to its extremes of 
travel. The dashpotrods must be adjusted so that when the dash- 
pot piston is at its lowest position, the hooks (see Eig. 59) descend 
just far enough for the hook dies to snap over the stud dies with 
about -gY" to to spare, depending on the size of the gear. 

To adjust and equalize the cut-off, lift the governor to about 
the position that it will occupy when running at normal speed, 
and put a block under the collar to hold it in this position. First 
set the double lever at the right of the governor cam rods so that 
it makes approximately equal angles with each rod, and then turn 
the engine over by hand until the piston has moved to the desired 
point of cut-off. Adjust the proper cam rod until the knock-off 
cam strikes the hook and allows the valve to close, then turn 
the engine to the point of cut-off on the other stroke and adjust the 
other cam rod in a similar manner. Now set the governor in the 
lowest position to which it could fall if there were no load on 
the engine, and set the safety cams so that in this position the hook 
cannot open the valve. A latch is provided on which the governor 
can be supported slightly above its lowest position so that the valves 
can be opened by the hook when starting the engine. As soon as 
the engine speeds up this latch must be moved aside, so that if the 
governor fails to act, it can drop to its lowest point, otherwise this 
latch would hold it just high enough so that the safety cams 
could not act. 

When Corliss gears are set as here described, the position of 
the eccentric may not be quite right, due to an incorrect estimate 
of the amount of lead required. The error is likely to produce 
faulty release and compression as well as poor admission, but it 
cannot be very serious, and the engine will turn over with its own 
steam, so that indicator diagrams may be taken. The final adjust¬ 
ments can then be determined from an examination of the diagrams. 




EXAMINATION PAPER 






























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VALVE GEARS. 


Read Carefully: Place your name and full address at the head of the 
Paper. Any cheap, light paper like the sample previously sent you may he 
used. Do'not crowd your work, but arrange it neatly and I egibly. Do not 
copy the answers from the Instruction Paper; use your own morels , so that 
we may he sure that you understand the subject. 


1. "Why cannot the displacement of the piston be found in 
the same manner as that of the valve ? 

2. What is the throw of the eccentric if the valve travel is 
3 inches and the rocker arm has the following dimensions: AB = 
12 inches, AG = 15 inches. See Figs. 19 and 20. 

3. Explain why the eccentric radius is not at right angles 
to the crank. What is lead ? 

4. Why are laps used ? What effect does an increase of 
outside lap have upon steam distribution ? 

5. Describe an eccentric. 

6. v Sketch a plain slide valve and indicate ports, bridges, 
exhaust cavity, inside and outside lap. 

7. Give the advantages of the Corliss valve gear. 

8. With a connecting rod of ordinary length, explain why 
there is unequal steam distribution, if the valve is set for one end 
and if the laps are equal. 

9. How do the following changes affect steam distribution 
with a plain slide valve, a. Increase in angular advance. 1. 
Increase in eccentricity. 

10. Describe the'‘essential features of the Joy valve gear. 

11. Referring to Fig. 16, will the line RO be perpendicular 
to XY when the piston is in the middle of the stroke ? 

12. What should be the diameter of the steam pipe for an 
18" X 24" engine that makes 100 revolutions per minute ? 

Ans. 5 inches. 

13. Describe how the governor regulates the engine by 
means of the shifting eccentric. 






VALVE GEARS 


14. When setting the Meyer valve, is the main valve given 
a late or early cut-off ? Why ? 

15. What relation between cut-off and compression can be 
obtained with the Meyer valve that is impossible with the ordi¬ 
nary slide valve? 

16. Describe, with sketch, the Stephenson link. 

17. Explain why an engine runs in the opposite direction if 
the valve is disconnected from one eccentric and connected to the 
opposite one. 

18. What should be the width of the exhaust port ? 

19. What is the advantage of the piston valve ? 

20. What are the advantages of the radial valve gears? 

21. What is one disadvantage of the Corliss gear? 

22. How may a valve be most easily balanced to overcome 
weight of valve and gear as well as steam pressure ? 

23. How does the Gooch link differ from the Stephenson 

link? 

24. Given in a certain valve: 

Yalve travel = 3 inches. 

Lead angle = 6°. 

Cut-off at | stroke. 

Compression at | stroke. 

Ratio of connecting rod to crank 4. 

Find by means of Zeuner’s diagram: 

Linear lead. 

Outside lap. 

Inside lap. 

Angular advance. 

25. What are 66 crossed rods”? How is steam distribu¬ 
tion with crossed rods different from that with open rods? 

26. How does the passage in the trick valve affect lead? 
How does it affect admission? 

27. For what two reasons are link motions used on locomo¬ 
tives ? 

28. Why does bringing the block nearer the center of the 
Stephenson link cause cut-off to occur earlier ? 

29. What advantage has the double-ported slide valve ? 

30. What is meant by the mid-position of a valve ? 




VALVE GEARg 


31. What is meant by an inside clearance when referring to 
the lap of valves ? 

32. Why is “compression” a desirable feature in the steam 
distribution ? 

33. How is the plain slide valve set for equal cut-off? 

After completing the work add and sign the following statement 0 

I hereby certify that the above work is entirely my own, 

(Signed) 

















































































































































































































































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NOV 2 1906 






















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